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    <td width="85%"> <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>The 
      Lazy Parser</b></font></td>
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<p>Closures are cool. It allows us to inject stack based local variables anywhere 
  in our parse descent hierarchy. Typically, we store temporary variables, generated 
  by our semantic actions, in our closure variables, as a means to pass information 
  up and down the recursive descent.</p>
<p>Now imagine this... Having in mind that closure variables can be just about 
  any type, we can store a parser, a rule, or a pointer to a parser or rule, in 
  a closure variable. <em>Yeah, right, so what?...</em> Ok, hold on... What if 
  we can use this closure variable to initiate a parse? Think about it for a second. 
  Suddenly we'll have some powerful dynamic parsers! Suddenly we'll have a full 
  round trip from to <a href="../phoenix/index.html">Phoenix</a> and Spirit and 
  back! <a href="../phoenix/index.html">Phoenix</a> semantic actions choose the 
  right Spirit parser and Spirit parsers choose the right <a href="../phoenix/index.html">Phoenix</a> 
  semantic action. Oh MAN, what a honky cool idea, I might say!!</p>
<h2>lazy_p</h2>
<p>This is the idea behind the <tt>lazy_p</tt> parser. The <tt>lazy_p</tt> syntax 
  is:</p>
<pre>    lazy_p<span class="special">(</span>actor<span class="special">)</span></pre>
<p>where actor is a <a href="../phoenix/index.html">Phoenix</a> expression  that 
  returns a Spirit parser. This returned parser is used in the parsing process. 
</p>
<p>Example: </p>
<pre>    lazy_p<span class="special">(</span>phoenix<span class="special">::</span>val<span class="special">(</span>int_p<span class="special">))[</span>assign_a<span class="special">(</span>result<span class="special">)]</span>
</pre>
<p>Semantic actions attached to the <tt>lazy_p</tt> parser expects the same signature 
  as that of the returned parser (<tt>int_p</tt>, in our example above).</p>
<h2>lazy_p example</h2>
<p>To give you a better glimpse (see the <tt><a href="../example/intermediate/lazy_parser.cpp">lazy_parser.cpp</a></tt>), 
  say you want to parse inputs such as:</p>
<pre>    <span class=identifier>dec 
    </span><span class="special">{</span><span class=identifier><br>        1 2 3<br>        bin 
        </span><span class="special">{</span><span class=identifier><br>            1 10 11<br>        </span><span class="special">}</span><span class=identifier><br>        4 5 6<br>    </span><span class="special">}</span></pre>
<p>where <tt>bin {...}</tt> and <tt>dec {...}</tt> specifies the numeric format 
  (binary or decimal) that we are expecting to read. If we analyze the input, 
  we want a grammar like:</p>
<pre><code><font color="#000000"><span class=special>    </span><span class=identifier>base </span><span class="special">=</span><span class=identifier> </span><span class="string">&quot;bin&quot;</span><span class=identifier> </span><span class="special">|</span><span class=identifier> </span><span class="string">&quot;dec&quot;</span><span class="special">;</span><span class=identifier>
    block </span><span class=special>= </span><span class="identifier">base</span><span class=special> &gt;&gt; </span><span class="literal">'{'</span><span class=special> &gt;&gt; *</span><span class="identifier">block_line</span><span class=special> &gt;&gt; </span><span class="literal">'}'</span><span class=special>;
    </span>block_line <span class=special>= </span><span class="identifier">number</span><span class=special> | </span><span class=identifier>block</span><span class=special>;</span></font></code></pre>
<p>We intentionally left out the <code><font color="#000000"><span class="identifier"><tt>number</tt></span></font></code> 
  rule. The tricky part is that the way <tt>number</tt> rule behaves depends on 
  the result of the <tt>base</tt> rule. If <tt>base</tt> got a <em>&quot;bin&quot;</em>, 
  then number should parse binary numbers. If <tt>base</tt> got a <em>&quot;dec&quot;</em>, 
  then number should parse decimal numbers. Typically we'll have to rewrite our 
  grammar to accommodate the different parsing behavior:</p>
<pre><code><font color="#000000"><span class=identifier>    block </span><span class=special>= 
            </span><span class=identifier>&quot;bin&quot;</span> <span class=special>&gt;&gt; </span><span class="literal">'{'</span><span class=special> &gt;&gt; *</span>bin_line<span class=special> &gt;&gt; </span><span class="literal">'}'</span><span class=special>
        |   </span><span class=identifier>&quot;dec&quot;</span> <span class=special>&gt;&gt; </span><span class="literal">'{'</span><span class=special> &gt;&gt; *</span>dec_line<span class=special> &gt;&gt; </span><span class="literal">'}'</span><span class=special>
        ;
    </span>bin_line <span class=special>= </span><span class="identifier">bin_p</span><span class=special> | </span><span class=identifier>block</span><span class=special>;
    </span>dec_line <span class=special>= </span><span class="identifier">int_p</span><span class=special> | </span><span class=identifier>block</span><span class=special>;</span></font></code></pre>
<p>while this is fine, the redundancy makes us want to find a better solution; 
  after all, we'd want to make full use of Spirit's dynamic parsing capabilities. 
  Apart from that, there will be cases where the set of parsing behaviors for 
  our <tt>number</tt> rule is not known when the grammar is written. We'll only 
  be given a map of string descriptors and corresponding rules [e.g. ((&quot;dec&quot;, 
  int_p), (&quot;bin&quot;, bin_p) ... etc...)].</p>
<p>The basic idea is to have a rule for binary and decimal numbers. That's easy 
  enough to do (see <a href="numerics.html">numerics</a>). When <tt>base</tt> 
  is being parsed, in your semantic action, store a pointer to the selected base 
  in a closure variable (e.g. <tt>block.int_rule</tt>). Here's an example:</p>
<pre><code><font color="#000000"><span class=special>    </span><span class=identifier>base 
        </span><span class="special">=</span><span class=identifier> str_p</span><span class="special">(</span><span class="string">&quot;bin&quot;</span><span class="special">)[</span><span class=identifier>block.int_rule</span> = <span class="special">&amp;</span>var<span class="special">(</span><span class="identifier">bin_rule</span><span class="special">)] 
        | </span><span class=identifier>str_p</span><span class="special">(</span><span class="string">&quot;dec&quot;</span><span class="special">)[</span><span class=identifier>block.int_rule</span> = <span class="special">&amp;</span>var<span class="special">(</span><span class="identifier">dec_rule</span><span class="special">)]
        ;</span></font></code></pre>
<p>With this setup, your number rule will now look something like:</p>
<pre><code><font color="#000000"><span class=special>    </span><span class=identifier>number </span><span class="special">=</span><span class=identifier> lazy_p</span><span class="special">(*</span><span class=identifier>block.int_rule</span><span class="special">);</span></font></code></pre>
<p>The <tt><a href="../example/intermediate/lazy_parser.cpp">lazy_parser.cpp</a></tt> 
  does it a bit differently, ingeniously using the <a href="symbols.html">symbol 
  table</a> to dispatch the correct rule, but in essence, both strategies are 
  similar. This technique, using the symbol table, is detailed in the Techiques section: <a href="techniques.html#nabialek_trick">nabialek_trick</a>. Admitedly, when you add up all the rules, the resulting grammar is 
  more complex than the hard-coded grammar above. Yet, for more complex grammar 
  patterns with a lot more rules to choose from, the additional setup is well 
  worth it.</p>
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<p class="copyright">Copyright &copy; 2003 Joel de Guzman<br>
  Copyright &copy; 2003 Vaclav Vesely<br>
  <br>
  <font size="2">Use, modification and distribution is subject to the Boost Software
    License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
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